Add ActivityIndicator for the web factories

Prior to this change, ActivityIndicator wasn't part
of the factory for the web example apps.

This change imports and adds it so that it can be
tested out.

web/src/toga_web/factory.py

@@ -8,6 +8,7 @@
 
 # from .images import Image
 from .paths import Paths
+from .widgets.activityindicator import ActivityIndicator
 from .widgets.box import Box
 from .widgets.button import Button
 
@@ -64,6 +65,7 @@ def not_implemented(feature):
     # 'OptionContainer',
     # 'PasswordInput',
     "ProgressBar",
+    "ActivityIndicator",
     # 'ScrollContainer',
     # 'Selection',
     # 'Slider',